1. Field of the Invention
This invention relates to an image forming apparatus which causes a beam of light to sweep a surface by the rotation of a rotary polygonal-prism mirror and a method for image formation with the apparatus. More particularly, this invention relates to an image forming apparatus which causes the high-frequency jitter caused mainly by errors in the angular division of the aforementioned polygonal-prism mirror to be eliminated in the central part of a formed image and a method for image formation with the apparatus.
2. Description of the Prior Art
The conventional image forming apparatus is designed to form an image by modulating a laser beam emitted from a laser beam oscillator with an image signal and polarizing the modulated laser beam with a rotary polygonal-prism mirror such as a polygon mirror thereby causing the laser beam to scan a recording medium.
The rotary polygonal-prism mirror which is used in the image forming apparatus of this operating principle plays an important role in the formation of an image and enjoys many advantages such as a large polarizing angle and inability to cause spectral diffraction. It is nevertheless susceptible of errors in angular division because it is not in the least easily fabricated with high accuracy.
When these errors in angular division are present, the starting position and the ending position of the scanning line of the laser beam are varied from one mirror surface to another and the array of picture elements in the direction of scanning inevitably deviates from one scanning line to another.
Once this deviation occurs, the image of a straight line that the apparatus is caused to draw perpendicularly to the direction of scanning is expressed not as a strictly straight line but as a periodically undulating curve from a microscopic point of view, for example.
For the purpose of eliminating the inconvenience, there has been followed the practice of disposing a sensor near the position for starting the scanning with the laser beam thereby ensuring coincidence of the starting position S of scanning lines as illustrated in FIG. 6 and allowing the issuance of a video signal to start after the sensor's detection of the laser beam thereby attaining alignment of the positions for starting image formation.
In the conventional apparatus embodying this practice, the positions S for starting image formation may be indeed aligned in all of the scanning lines 1 to 8 (where the rotary polygonal-prism mirror has 8 mirror surfaces) produced severally by the individual mirror surface of the rotary polygonal-prism mirror, the ending positions E of the scanning lines cannot be aligned in the presence of errors in plane division as illustrated in FIG. 6. This misalignment inevitably entails a maximum deviation P or high-frequency jitter at the ending positions. This high-frequency jitter periodically appears after each complete rotation of the rotary polygonal-prism mirror.
With reference to the concrete example cited above, the image of a straight line perpendicular to the direction of scanning is expressed as a straight line near the starting positions S at which the video signal is delivered to the scanning lines, whereas the same image is prevented from being expressed as a straight line near the ending positions E by the phenomenon of jitter. In other words, the deviations in the arrays of picture elements in the scanning lines gradually increase in the direction of scanning from the starting positions S to the ending positions E, with the inevitable result that the produced image will be disfigured as a whole in the direction of scanning.